In order to reduce a fatigue of a driver and safely drive, a motor-driven power steering apparatus is generally mounted on the vehicle. The motor-driven power steering apparatus is structured such as to assist a load of the steering shaft and energize via a transmission mechanism such as a gear of a speed reduction gear or the like on the basis of a driving force of the motor.
As a conventional motor-driven power steering apparatus, an outline structure of the steering system is generally shown in FIG. 1. In the drawing, a steering shaft 1 having a steering wheel in a leading end (a right end in FIG. 1) is rotatably supported by a ball bearing 3 within a coaxial steering column 2, and is extended in an axial direction. The steering shaft 1 is constituted by a tubular outer shaft 4, and an inner shaft 5 fitted within the outer shaft 4. Further, the steering column 2 is formed by connecting a tubular outer column 6 and an inner column 7 press fitted and fixed within the outer column 6. Further, when an impact load is applied in a compression direction at a time of colliding, the outer shaft 4 and the outer column 6 are pressed into a base end side (a left side in FIG. 1), absorbs an energy by contracting an entire length and absorbs an impact applied to a body of the driver colliding with the steering wheel.
Further, an input shaft 9 and an approximately tubular output shaft 10 are connected to the base end side (the left side in FIG. 1) of the inner shaft 5 via a torsion bar 8. The torsion bar 8 is inserted into the output shaft 10, one end of the torsion bar 8 is press fitted and fixed to the input shaft 9, and the other end thereof is fixed to the output shaft 10 by a pin 11.
Further, a speed reduction gear unit 12 is supported to an outer periphery of a center portion of the output shaft 10 by a pair of ball bearings 13 and 13. The speed reduction gear unit 12 is constituted by a worm wheel 14 fixedly mounted to an outer periphery of the output shaft 10 in accordance with a press fitting, a worm 15 engaging with the worm wheel 14, and a motor in which the worm 15 is mounted to an output shaft 16, and is structured such as to reduce a speed of rotation of the motor via the worm 15 and the worm wheel 14 so as to transmit a torque, on the basis of driving the motor.
Further, a torque sensor 17 is arranged in a leading end side (a right side in FIG. 1) of the speed reduction gear unit 12, and the torque sensor 17 is provided with the torsion bar 8 and an electromagnetic yoke 20 receiving a coil winding 19 in an outer periphery of a spline groove 18 formed in a leading end of the output shaft 10, and is structured such as to detect a magnetic change by the coil winding 19 within the electromagnetic yoke 20, by generating a torsion angle in correspondence to a torque generated in the steering shaft 1.
Further, a rotation angle sensor (a steering sensor) 21 is arranged in a base end side (a left side in FIG. 1) of the speed reduction gear unit 12, and the rotation angle sensor 21 is constituted by a tubular hollow member 22 arranged in an outer periphery of the output shaft 10, and a casing 23 rotatably supporting the hollow member 22. In this case, in the hollow member 22, a projection 24 is extended to an inner side from an inner peripheral surface, and is engaged with a locking hole 25 provided in an outer peripheral surface of the output shaft 10, whereby the hollow member 22 is integrally rotated with the output shaft 10. Accordingly, the structure is made such as to detect the rotation angle of the output shaft 10 by detecting a relative displacement between the casing 23 and the hollow member 22 by a detecting means 26 provided in the casing 23. Accordingly, the steering state of the steering wheel is detected from the rotation angle (the steering angle).
In this case, reference numeral 27 denotes a universal joint for connecting to an intermediate shaft, and reference numeral 28 denotes a bracket for mounting the steering apparatus to the vehicle body.
In this case, in order to protect a passenger at a time when the vehicle is collided, it is necessary that an energy absorbing mechanism is provided in the steering column 2 for the regulation or the safety. Accordingly, in the conventional structure mentioned above, the steering shaft 1 and the steering column 2 are respectively constituted by two members (the outer shaft 4 and the inner shaft 5, and the outer column 6 and the inner column 7), and the outer shaft 4 and the outer column 6 can be moved at a certain range (a stroke t) in the axial direction at a time of collision. Therefore, the steering column 2 or the like is plastically deformed at a time of moving, and it is possible to absorb the energy generated at a time when the passenger collides with the steering wheel, on the basis of the deformation energy.
In this case, since the absorbing amount of the energy is determined by a product of a force applied by the impact and the stroke t, it is important to make the stroke t as long as possible in order to make the impact force to the passenger small so as to reduce an injury.
However, in the column type motor-driven power steering apparatus, it is necessary that the speed reduction gear and the torque sensor 17 are provided in the axial direction of the steering shaft 1, and it is necessary that the rotation angle sensor 21 is provided so as to have such a space that the steering shaft, that is, the output shaft 10 is exposed between the universal joint 27 and the speed reduction unit 12. Accordingly, the stroke t is limited to a fixed length from the space of the vehicle body, and there is a problem that it is hard to secure the stroke t of the energy absorbing mechanism to a sufficient length in the limited space.
Accordingly, an object of the present invention is to provide a motor-driven power steering apparatus in which a rotation angle sensor can be attached to a steering shaft in the limited space without deteriorating an energy absorbing function for protecting the passenger.